Recently, electronic cameras have been used instead of conventional film-based cameras to take still pictures and motion pictures in monochrome and full color. In an electronic camera, light from the subject is focused on an image sensor. The image sensor is normally an array of charge-coupled optical detector elements, each of which includes a light detector and a charge storage device. The gate-to-substrate capacitance of an MOS transistor may be used as the charge storage device.
The light detector generates charge in proportion to the amount of light falling on it, and the charge is temporarily accumulated in the charge storage device. Then, a charge transport system transfers the accumulated charge from the charge storage device in each of optical detector elements to an image data generator. The image data generator generates image data representing the units of charge it receives. The image data output by the image data generator are subject to image processing and the set of data representing a picture of the subject is stored in a suitable memory. The charge transport system is typically a charge-coupled device (CCD). CCDs have superior performance, and are used in still picture and video cameras that take monochrome or color pictures
The image sensor of an electronic camera and the photographic film of a film-based camera can both be regarded as light-sensitive elements. The light-sensitive element of an electronic camera and that of a film-based camera interact with light in basically the same way. The picture taken by the camera is the result of all of the light that falls on the light-sensitive element during the exposure. The light that falls on the light-sensitive element includes not only the light from the subject, but also additional light emitted or reflected by other objects in the field of view of the camera. Such other objects in the field of view of the camera constitute the background of the picture.
Pictures obtained from electronic cameras are in many ways similar to those obtained from film-based cameras. For example, to extract the subject and to eliminate the background in a picture generated by an electronic or film-based camera, the picture must be input into a computer as digital picture data. Then, graphics software is used to process the picture data to generate new picture data that represent the subject and not the background. A new picture containing the subject and not the background is then printed or displayed in response to the new picture data. If such processing is to be performed in real time on a motion picture (video) input and the subject is to be displayed in real time, a large-scale integrated circuit capable of performing high-speed image processing is required. A system to perform such processing is more complex and is more expensive to manufacture than the electronic camera itself
In addition, pictures obtained from conventional electronic and film-based cameras suffer from many of the same defects. For example, the subject may be over-exposed in a picture taken by a conventional camera of a strongly-illuminated subject against a dark background. In another example, the subject may be under-exposed in a picture taken by a conventional camera of a subject under strong back lighting. Over- or under-exposure of the subject makes the picture unsatisfactory.
Furthermore, a special circuit is usually provided in an electronic camera to reduce or eliminate picture noise caused by the dark current of the light detectors of the image sensor. This special circuit increases the cost and complexity of the already-complex processing circuits in a conventional electronic camera.
What is needed is an electronic camera that can take pictures in which only the subject appears, that can take pictures in which the subject is enhanced relative to the background, and that can enhance a desired hue in the picture without the need to use a computer or other device to perform complex image processing.
What is also needed is an electronic camera that can eliminate or reduce the effects of backlighting and that can take properly-exposed pictures of the subject under backlighting conditions.
Finally, what is needed is an electronic camera in which picture noise caused by dark currents in the image sensor is reduced or eliminated without the need for complex additional circuitry.